Methods of processing waste liquid with a waste liquid processing system comprising a body of waste liquid, and a floating filter disposed above this waste liquid body and consisting of floating plastic pieces having a specific weight less than that of water are described in U.S. Pat. No. 4,115,266 issued Sep. 19, 1978 to the Inventor of the present application.
The organic waste liquids from the fast food and restaurant industry, the marine product processing industry, and the meat production industry generally contain animal and vegetable oil, and their BOD and COD levels are typically at the extremely high levels of 5,000 to 10,000 mg/liter. Batch processes, lagoons and aeration are typically employed for cleaning such high concentration organic waste liquids. However, in carrying out such processes, the biological process is hampered by the presence of large amounts of oil matters, and the pre-processing such as aggregation/precipitation and/or floatation separation process are therefore indispensable. This calls for a need for large scale facilities, and causes an increase in the running cost. Further, the process is too complex for it to be used in small-scale facilities such as sewage processing systems intended for individual household use.
The methods described in the aforementioned U.S. patent account for organic waste liquids containing large amounts of oil matters, but do not provide that the waste liquid is allowed to move at a speed suitable for anaerobic processes. On the contrary, aeration was considered necessary for promoting aggregation of foreign substances on the particles forming the floating filter layer.